1. Field of the Invention
The present invention relates in general to wastewater collections systems for wastewater treatment plants, and is particularly directed to pump management, wastewater treatment plant inflow management, sediment elimination, pressure management and septic elimination in wastewater collection systems, wherein an improvement to the operation of wastewater collection systems is made by modulating the operation of the pumping stations systematically, rather than allowing pumps to operate independently.
2. Description of the Prior Art
In a wastewater collection service area, many pump stations often pump wastewater to a treatment facility or to a master pump station through a force main. Force mains are used to move wastewater under pressure, typically over terrain that is not conducive to gravity flow. A typical wastewater pump station may house two, three or more pumps. The number and size of the pumps at each station are determined by the amount of wastewater inflow to that site and by amount of storage capacity in a wet well. There is normally no coordination of activity between pumping stations. Pump operations at a given wastewater pump station are typically controlled locally. When the influent in the wet well at a pumping station reaches a certain predetermined level, a sensor triggers the “lead” (or first) pump to start. If the pump's action is not sufficient to handle the inflow of sewage, additional, or “lag” pumps, are called into service. When the wet well level lowers to the desired level, another sensor causes the pump(s) to turn off.
Most wastewater is generated during two or three peak periods of daily activity, mostly in the morning when families are rising for the day and in the early evening when families are returning home from work and school. Throughout the operating day, but especially during these peak periods, it is normal for many stations to pump simultaneously into the same force main since there is no coordination between pumping stations. The more stations that pump at one time, the higher the head pressure in the force main becomes. Pumps cannot pump efficiently when pumping against higher pressures—smaller pumps may not be able to pump at all when more powerful pumps are dominating the force main—causing pumps to run longer in order to accomplish their purpose. This condition results in drastically higher energy costs, increased wear and tear on pumps and mains and a greater likelihood of failures that cause sewage spills. Conversely, there are also many periods during which no pumps are running. When wastewater generation is at the lowest volume, in the middle of the night or, perhaps at mid-morning or mid-afternoon, the majority of pumping stations are frequently dormant.
Wastewater treatment plants are designed and sized one of two ways: either to accommodate estimated daily total inflows, or to serve peak demands on the system. Plants designed to accommodate a daily average inflow run the risk of being incapable of handling those few peak hours of the day when most of the inflow occurs. Those designed to accommodate the peak inflow hours sit idle for most of a twenty-four hour period, since the peak inflows only occur during a few hours of the operational day. Failure to provide sufficient capacity will likely result in pretreatment spills, damage to infrastructure with attending high repair costs and loss of service to customers and damage to customer property. This consequence forces careful sizing to accommodate the long-term estimate of peak loading. The daily capacity of the infrastructure to treat wastewater may be supplemented by surge tanks either in the initial design or as an addition to the existing infrastructure. In either case, the infrastructure and surge tanks are quite costly and add to on-going maintenance costs.
Very frequently, wastewater treatment plants are sized based on estimated daily total inflows and are measured in terms of (millions) of gallons per day. When, over a period of years, population increases exceed long-term plans, the addition of surge tanks to temporarily store the unmanageable inflow is necessary to prevent overflows at the plant's headworks, the point of inflow into the plant. Later, during lower flow times, surge tank contents may be processed through the treatment system. Surge tanks provide temporary storage at the wastewater treatment plant to accommodate the wastewater plant's processing rate, but do not lessen flow into the infrastructure.
Wastewater collection systems have pump stations distributed throughout the service area that arbitrarily pump wastewater to the treatment plants. But these pumps only respond to local conditions and are activated by local level sensors without regard to events or conditions elsewhere. As a result, during peak activity hours, e.g., from 6 AM to 8 AM and from 5 PM to 7 PM, these pump stations operate more frequently, thereby raising the inflow into the plant, frequently surpassing the plant's processing capability. It should be noted that these high inflow periods normally occur between periods during which there is very little, if any, flow into the wastewater treatment plant. The result is normally a need for a higher design margin and/or earlier system expansion.
Another consequence of unmanaged flow is reduced efficiency in managing the biological treatment material at the wastewater treatment plant. The most efficient biomass management requires a relatively constant flow rate so that bacteria used in processing can effectively interact with the influent waste. High or irregular flow rates lessen the efficiency of the treatment system and the quality of the treated effluent.
Sewer mains are frequently sized to take into account future growth in a wastewater collection service area, but it is not unusual for years to pass before the volume of wastewater reaches the level for which the system was designed. During this time, the utility may experience maintenance problems if the volume of wastewater through the main is not sufficient to carry with it solids and silt. If the flow of wastewater is so small relative to the pipe size that it fails to reach a velocity known as ‘scour speed’, solids will fall out of the liquid and accumulate in the bottom of the sewage mains. When this occurs, expensive specialty vehicles typically known as vacuum trucks are employed to clean out sediment that accumulates in the sewage system.